A negative glow discharge lamp typically is comprised of an hermetically sealed light-transmitting envelope in which are located a cathode and an anode supported by associated lead-in wires. A phosphor coating is provided on the inner surface of the lamp envelope. A quantity of fill gas and mercury are introduced into the lamp during manufacturing. An example of a such a negative glow discharge lamp is described in U.S. Pat. No. 4,904,900, which issued to Bouchard et al and is assigned to the same Assignee as the present application.
Operation of a typical glow discharge lamp is as follows. Electrical heating of the cathode causes thermionic emission of electrons. These electrons are accelerated to about 15 electron volts of energy by the presence of a sheath of positive mercury ions about the cathode. These electrons impinge on gas and especially mercury atoms causing ionization with the release of secondary electrons. Both the primary and secondary electrons can excite mercury atoms such that they give up their acquired energy in the form of ultraviolet photons. It is these UV photons that strike the phosphor and cause it to emit visible light. Electrons enter the anode to complete the electrical circuit through the lamp.
Negative glow discharge fluorescent lamps differ principally from conventional positive column fluorescent lamps in that the electrons in the plasma thereof comprise entirely energetic primary electrons accelerated by the cathode fall and secondary electrons released by collision of those primary electrons with atoms in the gaseous phase of the lamp. There is no low voltage gradient positive column in which the electrons acquire a "drift" velocity.
In order to achieve reasonable discharge efficiencies in a negative glow discharge lamp (e.g., 30 lumens per watt or higher), it is necessary to use a fill gas that is not readily ionized or excited by the energetic primary electrons, which have energies on the order of 15 electrons volts. Argon, and the heavier inert gases, are excited by such electron energies and typically result in lumen losses of 40 percent or more compared to neon. Helium, which is the most resistant inert gas toward ionization or excitation, also has been found to promote serious lamp efficiency losses relative to neon by virtue of its low atomic mass and resulting high elastic scattering losses. Much test data has therefore shown that the use of neon as the fill gas is optimum for negative glow fluorescent lamps.
An undesirable consequence of the use of neon as the fill gas in such lamps is that any conditions that tend to elevate lamp operating voltage, as for example low ambient temperature, result in red neon excitation and a disturbing nonuniformity of color from lamp to lamp. Such color shift is often seen during lamp starting, and also occurs under certain dimming conditions. The ability to dim negative glow fluorescent lamps over essentially their full operating range is one of the attractive features of such lamps. A ballast supply that provides elevated voltage spikes in its waveform can promote ongoing neon excitation during lamp life. Additionally, the ability of the lamp to realize such elevations in voltage results in significant sputtering of the cathode coating and a reduction in lamp life relative to what the life could be if the discharge voltage were somehow more of less truncated at, for example, 14.5 volts.
Various quantities of krypton have been used in the fill of fluorescent glow lamps not containing mercury. For example, U.S. Pat. No. 2,409,769 teaches the use of preferably neon when cadmium borate is used in the coating to give red or pink light. According to the patent, the characteristic red light produced by the neon will supplement that produced by excitation of the coating. A small amount of krypton (i.e., less than one percent) is added to the neon.
U.S. Pat. No. 2,421,571 relates to another fluorescent glow lamp without mercury. The gaseous atmosphere taught by the patent consists of true inert gases which do not condense at ambient or operating temperatures. According to the patent, when atmospheres of certain materials such as mercury vapor are used, the life and efficiency of the lamp is greatly impaired by condensation which blocks out a substantial portion of the visible light emitted from the lamp. In one example of pink and yellow lamps, a gaseous mixture of 95 to 99 percent neon and the rest krypton is taught.
Although the use of krypton in standard positive column fluorescent lamps is well known, its use in negative glow type discharge lamps of high desired lumen output (e.g., over 100 lumens) has been considered detrimental to those skilled in the art of negative glow discharge lamp making.